This invention relates to radar signal processing apparatus and, more particularly, an adaptive radar signal processing apparatus, in which undesired signals are suppressed in an adaptive radar signal processing by switching filter characteristics according to various regions responsible for generation of undesired signals.
Generally, according to the theory of digital filters, the input response of a digital filter having fixed filter coefficients is expressed as ##EQU1## were X(l) represents an input signal, W(l) is a filter constant and Y(m) is an output signal. Here, m, l and N are integers, N represents the number of digital filter stages, and the input/output signal and filter coefficients are complex numbers.
FIG. 1 shows the structure of a conventional digital filter. In this case, signals involved are all complex numbers, and arithmetic units are all complex number arithmetic units. Referring to FIG. 1, the circuit illustrated comprises an input terminal 100, memories 111-1, 111-2, ---, 111-(N-1), weighting multipliers 121-1, 121-2, ---, 121-N, filter coefficient application terminals 131-1, 131-2, ---, 131-N, an adder 140 and an output terminal 150. The memories have a memory capacity required to store all reflected signals (digital signals) which are obtained, like radar reception signals, successively for each transmission pulse.
The frequency response characteristics of a digital filter are determined definitely by specifying the filter coefficient (i.e., W(l) in FIG. 1), and various digital filters can be realized which have an MTI (Marring Target Indication) cancellation function and a DFT (Dispersive Fourier Transform) function in radars to obtain desired response characteristics within the range of the freedom of the filter (i.e., number N of filter stages).
Incidentally, the radar reception signal contains a signal reflected from the ground, buildings, etc. (referred to as stationary clutter), a signal reflected from rain droplets, clouds, etc. (referred to as moving clutter), a signal reflected from an aircraft (referred to as target or desired signal) and a noise signal. The stationary clutter has a Doppler spectrum centered on zero Doppler frequency. The moving clutter and desired signals respectively cover a wide range of frequencies about the zero Doppler frequency.
Doppler spectra of these signals are exemplified in FIG. 2 where 200 represents a stationary clutter spectrum, 210 a moving clutter spectrum, and 220 a spectrum of the desired signal. The stationary and moving clutters (both being simply referred to as clutter) and the noise signal stand for undesired signals in the radar reception signal and generally, the former is a more intensive than the latter.
Therefore, a filter characteristic is desired, which can sufficiently suppress the undesired signal attributable to the clutter region to maximize the degree of S/C improvement (i.e., the degree of input/output improvement in the ratio of the desired signal to the clutter). More specifically, since a signal representative of the stationary clutter attributable to the stationary clutter region is strong, it is necessary for the filter to have, for the purpose of suppressing the stationary clutter signal, a filter characteristic which has null filtering at the zero frequency point and has sufficiently low side lobe levels in the neighborhood of the zero frequency so that sufficient suppression can be obtained. For the moving clutter region, since the moving clutter prevails over a wide range, it is necessary to provide a uniformly low side lobe characteristic. For a region where there is only a noise signal (i.e., clear region), a filter characteristic is desired which maximize the S/N improvement degree (i.e., the degree of input/output improvement in the ratio of the desired signal to the noise), and hence a DFT filter is requireed.
In the prior art digital filter, however, the filter coefficient has been fixed, and the filter has an MTI cancellation function, a DFT function, a digital filtering function or a function of MTI cancellation and digital filtering in combination. The MTI cancellation is effective for the stationary clutter region but is not suited to a composite clutter region covering both the stationary clutter and moving clutter. The DFT is best suited to the clear region with the maximum S/N improvement degree. However, the DFT fails to provide necessary suppression for the stationary and moving clutter regions since the side lobe is high in these regions. The combination of the MTI cancellation and digital filtering is effective for the stationary and moving clutter regions, but is not suited for the clear region since the maximum S/N improvement degree cannot be obtained. In the case of the digital filters, the filter characteristic can be so determined as to be suited for the stationary clutter region, moving clutter region and clear region, but the optimum filter characteristic is not the same in the individual regions. As has been explained, the prior art digital filter, because of its fixed filter coefficent, is not suited for some regions although suited for the other regions.